Factbox: Obama's Clean Power Plan faces tough legal scrutiny

WASHINGTON (Reuters) - The final version of the U.S. Environmental Protection Agency's Clean Power Plan, which aims to curb carbon emissions from power plants, is scheduled for release on Monday. The long-term fate of the measure depends on its ability to withstand the inevitable legal challenges from states and industries opposed to it.

Here are some questions and answers about how the legal fight could unfold:

Q: Challengers are first expected to ask a federal appeals court in Washington to block the regulation from taking effect until litigation is complete. Is a stay likely?

A: It is relatively rare for the U.S. Court of Appeals for the District of Columbia Circuit to grant stay requests of any type. The court did not block the Obama administration's first wave of climate regulations, and it recently allowed the Federal Communication Commission's new rules on Internet traffic to proceed.

However, in December 2011, the court did block an EPA air regulation to limit pollution crossing state lines.

To obtain a stay, petitioners must show likelihood that they will ultimately win the case and suffer "irreparable harm" if a stay is not granted. David Doniger, a lawyer with the Natural Resources Defense Council, counters that the challengers would have difficulty making a case for irreparable harm because the bulk of the regulation does not take effect immediately.

Q: Once the question of a stay is resolved, what oral arguments are the challengers expected to make?

A: Industry and state lawyers have said they believe the regulation will exceed the scope of the 45-year-old Clean Air Act, the law under which the EPA is acting. Critics say in part that the legislation only allows the government to regulate greenhouse gases spewed from "an existing source" such as individual power plants.

But the Clean Power Plan will allow states to measure emissions cuts from a variety of sources "outside the fenceline," including gains from other renewable energy sources and energy efficiency measures.

While courts usually defer to an agency's interpretation of a law, the challengers are expected to argue that this should not happen in this case. The Supreme Court has said that judges can depart from this practice in cases of "deep economic and political significance.".

Q: Which way is the appeals court likely to lean?

A: The fate of the regulation depends heavily on which randomly assigned three-judge panel hears the case. Until recently, the court had a majority of Republican-appointed judges. But Obama has appointed four of his own nominees in recent years, and Democratic appointees are generally viewed as more sympathetic to action by government agencies.

Q: Once the appeals court rules, the losing side will almost certainly seek a Supreme Court review. Is the high court likely to take the case?

A: The nine Supreme Court justices have shown considerable interest in the Obama administration's air pollution regulations, hearing three cases in the last two years.

The rulings have been mixed. In April 2014, the court upheld a regulation limiting air pollution across state lines. In June 2014, it largely upheld the government's ability to regulate greenhouse gas emissions from major utilities but did exempt some facilities that the agency wanted to regulate. Then, last month, the court ruled against the EPA for not considering compliance costs when it moved to limit emissions of mercury and other hazardous air pollutants, mainly from coal-fired power plants.

Q: What is the likely outcome?

A: Industry lawyers say the high court has become increasingly skeptical of large-scale agency rule-making. The EPA's "outside the fenceline" proposal could raise eyebrows among the court’s conservatives, who have already expressed concern about the government using the Clean Air Act as a tool to combat climate change.

In the 2014 carbon emissions case, Justice Antonin Scalia said that when an agency "claims to discover in a long-extant statute an unheralded power to regulate a significant portion of the American economy, we typically greet its announcement with a measure of skepticism."

But it was also the Supreme Court that ruled in the landmark Massachusetts v. EPA case in 2007 that the Clean Air Act allows the government to regulate carbon emissions. With the court closely divided on ideological lines, the outcome is too hard to call.